1. Field of the Invention
The present invention relates to a method of preventing polymer scale deposition on the inner wall and so forth of a polymerization vessel during polymerization of a monomer having an ethylenically double bond.
2. Description of Prior Art
In processes of preparing polymers by polymerizing a monomer in a polymerization vessel, the problem that polymers deposit on the inner wall and other parts which come into contact with the monomer such as stirring blades of the polymerization vessel in the form of scale, is known. The deposition of the polymer scale on the inner wall results in disadvantages that the yield of the polymer and the cooling capacity of the polymerization vessel are lowered; that the polymer scale may peel and mix into manufactured polymer, thereby impairing the quality of the manufactured polymer; and that removal of such polymer scale is laborious and hence time-consuming Further, since the polymer scale contains unreacted monomer, there is a danger that the workers may be exposed to the unreacted monomer, which may give them some physical disorders.
Heretofore, there are known methods for preventing polymer scale deposition in which the inner wall and so forth are coated with various materials as polymer scale preventive agents, for example, particular polar compounds (Japanese Patent Publication (KOKOKU) No. 30343/1970); dyes or pigments (Japanese Patent Publication (KOKOKU) No. 30835/1970); particular linear or branched polyaromatic amines (Japanese Patent Publication (KOKOKU) No. 16561/1984); condensates prepared by reacting an aromatic amine compound with an aromatic nitro compound in the presence of a condensation catalyst at 100.degree. to 250.degree. C., or the basic form thereof produced by treatment with an alkali metal salt or ammonium compound (Japanese Patent Publication (KOKOKU) No. 30681/1985); and a quinone-amine compound with an average molecular weight of 3,000 or more prepared by subjecting an aromatic amine and an aromatic quinone to addition reaction in a solvent having a solubility parameter of 9.0 to 12.2 or a mixed solvent containing said solvent and an alcohol (Japanese Pre-examination Patent Publication (KOKAI) No. 7309/1986); and a reaction product of a phenolic compound with an aromatic aldehyde (Japanese Pre-examination Patent Publication (KOKAI) No. 54317/1980).
Further, Japanese Pre-examination Patent Publication (KOKAI) No. 210902/1983 discloses a process of polymerization of a vinyl monomer in a polymerization vessel, which comprises coating the inner wall and other parts with which the monomer comes into contact of the polymerization vessel, with an aqueous solution comprising: (I) at least one compound selected from (i) alkali metal salts and ammonium salts of sulfonic acid type or carboxylic acid type dyes, (ii) alkali metal salts and ammonium salts of organic sulfonic acids or carboxylic acids having a conjugated double bond, (iii) alkali metal salts and ammonium salts of sulfonation products prepared by sulfonation of a condensate of an aromatic amine compound with an aromatic nitro compound or a self-condensate of an aromatic amine compound, and (iv) anionic polymer electrolytes; (II) an inorganic colloid; and, optionally, (III) a compound that can react with any of said component (I) and is selected from (a) dyes, (b) organic compounds having a conjugated double bond, and (c) cationic polymer electrolytes, and drying the coated surface.
These methods are effective in preventing polymer scale deposition, in the case where the monomer to be polymerized is vinyl halide such as vinyl chloride or a monomer mixture containing a large amount of vinyl chloride and a small amount of other monomers copolymerizable therewith. However, in the case where the monomer having an ethylenically double bond to be polymerized mainly comprises styrene, .alpha.-methylstyrene, acrylate, acrylonitrile or the like, which have so high a dissolving power against the coatings formed by the above prior art methods, the coatings may be partly or completely dissolved away. Consequently, it becomes impossible to prevent the deposition of polymer scale.
Japanese Pre-examination Patent Publication (KOKAI) No. 72902/1985 discloses a process of polymerization of a monomer having an ethylenically double bond in a polymerization vessel, wherein the inner wall and other parts of the polymerization vessel with which the monomer comes into contact during polymerization have been previously coated with a coating solution comprising: (a) a condensate of an aromatic amine compound with an aromatic nitro compound, and (b) an alkali metal salt or ammonium salt of the sulfonation product prepared by sulfonation of a condensate of an aromatic amine compound with an aromatic nitro compound, followed drying. This process has an improved scale preventing effect, but one polymerization run results in polymer scale deposition in an amount of about 3 to 8 g/m.sup.2. In particular, the polymerization of the monomers having high dissolving power such as styrene, .alpha.-methylstyrene, etc. results in much polymer scale; hence it is impossible to use the polymerization vessel continuously or repeatedly. That is, the operation of removing polymer scale has to be conducted every polymerization run.